We propose that the actions of parathyroid hormone (PTH) are modified at the cellular level by the PTH induction of cyclooxygenase-2 (COX-2) and COX-2-produced prostaglandins (PGs). PTH is a systemic hormone acting predominantly on osteoblast (OB) lineage cells, whereas PGs are autocrine/paracrine lipids acting on both OB and osteoclast (OC) lineage cells. PTH is a potent inducer of COX-2 and PGs in OBs. Although PTH is generally thought to be osteogenic or anabolic only when given intermittently, continuous exposure to PTH markedly stimulates OB differentiation in vitro in bone marrow stromal cell cultures when COX-2 expression or activity (PG production) is absent. An inhibitory interaction between PTH and COX-2 is supported by in vivo data showing that COX-2 knockout (KO) mice have increased anabolic responses to intermittent PTH compared to COX-2 wild type (WT) mice. Preliminary studies in vitro suggest that the inhibitory effects of the interaction of PTH and COX-2 on OB differentiation require the presence of OC precursors and the expression of the PGE2 receptor EP4R on OC precursors. We will test the hypothesis that PTH-stimulated OB differentiation in vitro and PTH-stimulated anabolic responses in vivo are inhibited by PTH- induced PGs acting via the EP4R receptor on OC precursors. For both in vitro and in vivo studies, we will use mice with global and targeted deletions of Cox-2, Ep4r, and Ep2r. Deletions will be targeted to the early OC lineage with CD11b-Cre, to the later OC lineage with cathepsin K-Cre (CtskCre/+), and to the early OB lineage with 3.6Col1a1-Cre. We will use the pattern of gene expression in osteogenic vs. inhibitory conditions to identify potential pathways, such as Wnt signaling, associated with the osteogenic effects. In vivo, we will examine anabolic and catabolic responses to intermittent and continuous PTH with radiographic and histomorphometric measures of skeletal phenotype, markers of bone turnover, and gene expression. This study addresses a novel role for PGs and OCs in the effects of PTH. It is an opportunity to explore mechanisms by which two agents, both of which can be anabolic, produce a negative interaction, in an effort to understand the specific pathways involved. It should lead to a better understanding of the roles of endogenous PGs in both OB and OC lineage cells and the differential effects of PGE2 receptors, and this knowledge may help to target bone remodeling more effectively to treat osteoporosis and other skeletal defects. The possibility that manipulation of endogenous PGs might increase the anabolic effects of PTH could have clinical applications.